Various types of dot matrix line printers have been proposed and are in use. In general, dot matrix line printers include a print head comprising a plurality of dot printing mechanisms, each including a dot forming element. The dot forming elements are located along a line that lies orthogonal to the direction of paper movement through the printer. Since paper movement is normally vertical, the dot forming elements usually lie along a horizontal line. Located on the side of the paper remote from the dot forming elements is a platen and located between the dot forming elements and the paper is a ribbon. During printing, the dot forming elements are actuated to create one or more dots along the print line defined by the dot forming elements. The paper is incremented forwardly after each dot row is printed. A series of dot rows creates a row of characters.
While the present invention was developed to shuttle the print head of a dot matrix line printer and, thus, is expected to find its primary use in such printers, it is to be understood that the invention can be used to shuttle the carriages of other mechanisms requiring or desiring precise, controlled velocity shuttle motion.
In general dot matrix line printers fall into two categories. In the first category are dot matrix line printers wherein only the dot forming elements are shuttled. In the second category are dot matrix line printers wherein the entire print head, e.g., the actuating mechanisms as well as the dot forming elements, are shuttled. Regardless of the type, the portions of the dot printing mechanisms to be shuttled are mounted on a carriage and the carriage is moved back and forth (e.g., shuttled) by a shuttling mechanism. The present invention is useful with both categories of dot matrix printers. More specifically, while the invention was developed for use in connection with a dot matrix line printer wherein the entire print head is shuttled, the invention can also be utilized with dot matrix line printers wherein only the dot forming elements are shuttled.
In the past, various types of carriage shuttling mechanisms have been proposed for use in dot matrix line printers. One such type of carriage shuttling mechanism includes a stepping motor that is connected to the carriage so as to cause step increments of carriage movement. At the end of each step, the appropriate actuating mechanisms are energized to create dots. Bidirectional printing is provided by stepping the carriage first in one direction and then in the opposite direction. A major disadvantage resulting from the use of stepping motors in dot matrix line printers, particularly dot matrix line printers wherein the actuating mechanisms as well as the dot forming elements are shuttled, is that conventionally sized stepping motors have insufficient power to move the print head of such dot matrix line printers. That is, while conventionally sized stepping motors have adequate power to shuttle only the dot forming elements, they are marginal at best in printers wherein the entire print head is shuttled. In addition, stepping motors have a speed limitation that makes them undesirable for use in relatively high speed dot matrix line printers, e.g., 600 and above lines per minute (lpm) dot matrix line printers.
As a result of the inherent limitations of stepper motor shuttle systems, attempts have been made to utilize constant speed AC and DC motors to shuttle the print head of dot matrix line printers. One of the major disadvantages of constant speed motor shuttling systems resides in the coupling mechanisms used to couple the motors to the print head. In most instances, the coupling medium is a cam and cam follower mechanism. Cam/cam follower mechanisms are undesirable in a dot matrix line printer shuttle system because they are subject to a high degree of mechanical wear. More specifically, dot matrix line printers, particularly high speed dot matrix line printers, require precision positioning of the printer head at the time the dot forming elements are actuated by their related actuating mechanisms. Mechanical wear is highly undesirable because it reduces the precision with which the print head can be positioned. As print head positioning precision drops, dot misregistration increases. As a result, printed characters and images are distorted and/or blurred. Distorted and/or blurred images are, of course, unacceptable in environments where high quality printing is required or desired. More specifically, in order to produce high quality printing, it is necessary for a dot matrix line printer to be able to precisely position dots at the same position in each dot line. If this result cannot be accomplished, the resulting images and characters are blurred and/or distorted.
Another disadvantage of many prior art carriage shuttling systems that include constant speed motors and cam/cam follower coupling mechanisms is that the displacement versus time curve that they produce is nonlinear. As a result, relatively sophisticated carriage position sensing and control sytems are required if precise dot positioning is to be achieved.
In order to avoid the mechanical wear factor and nonlinear carriage displacement versus time curve produced by prior systems for mechanically coupling a constant speed motor to the print head of a dot matrix line printer, a proposal has been made to use a coupling system that includes a pair of elliptical pulleys. See U.S. Pat. No. 4,387,642, entitled "Bi-Directional, Constant Velocity, Carriage Shuttling Mechanism" by Edward D. Bringhurst et al. While the bi-lobed, second order eliptical gear coupling mechanism described in this patent application has certain advantages over prior coupling mechanisms, it also has certain disadvantages. For example, it is undesirably noisy, mechanically complex and more expensive to manufacture than desirable.
In addition to stepping motor systems and constant speed motor systems, in the past, proposals have been made to use linear motors to shuttle the carriages of printer mechanisms. A linear motor is a motor wherein the axis of movement of the movable element of the motor is rectilinear rather than rotary. One such proposal is described in U.S. Pat. No. 3,911,814, entitled "Hammer Bank Move Control System" by Clifford J. Helms, et al. This patent describes a hammer bank system wherein the hammer bank is moved back and forth between two positions. In one position the hammers are aligned with odd character positions and in the other the hammer bank is aligned with even character positions. In response to control signals, the hammer bank is actuated to imprint a character when the appropriate character type is aligned with the hammer. In other words, this mechanism is directed for use in a character printer, as opposed to a dot matrix printer. Obviously, a character printer does not have the precise printer head positioning requirement of a dot matrix line printer.
One proposal to utilize a linear motor in a dot matrix line printer is described in U.S. Pat. No. 4,180,766, entitled "Reciprocating Linear Drive Mechanism" by Jerry Matula. In the system described in this patent, a reciprocable drive mechanism supporting the hammer bank is mounted to undergo free flights with low friction along a selected axis parallel to a printing line. At each limit of movement the drive mechanism encounters a resilient stop member which reverses the direction of motion of the drive mechanism and the hammer bank. Losses occurring during a reversal are compensated for by an energy impulse from a coupled linear electromagnetic drive and an associated velocity servo system, which eliminates the need for close servo control during reversal, allowing the drive mechanism to rebound naturally. During reversal, the velocity servo system, which is driven into saturation, senses the occurrance of zero motion of the drive mechanism and reverses the direction of energization of the electromagnetic drive. Hammer bank velocity during movement through a print span is sensed, and further kinetic energy is supplied by the servo system as required to compensate for friction losses, braking effects during printing, and other causes of variations in hammer bank speed.
There are a number of disadvantages to the reciprocating linear drive mechanism described in U.S. Pat. No. 4,180,766. For example, the use of a low power motor, primarily designed to overcome friction and printing loads, results in a system that has slow turnaround time, whereby overall printer speed is low. This undesirable result is enhanced by the use of a rebound system, as opposed to an energy storage system to improve turnaround time. Also, mechanism of the type described lin U.S. Pat. No. 4,180,766 consume several shuttle cycles before shuttle speed is raised to the desired printing speed. In other words, print start up time is high, which is particularly disadvantageous in printers that are operated in an intermittent manner.
A further example of a dot matrix line printer where a print head is reciprocated by a linear motor is the Model 2608A Line Printer produced by the Hewlett-Packard Company, Palo Alto, Calif. In this printer both the print head and the linear motor are supported by flexures. One disadvantage of this printer is an undesirably high level of vibration due to the difference in resonant vibration frequencies between the flexure supported print head mechanism and the flexure supported linear motor mechanism.